1. Field of the Invention
This invention relates to a process and process apparatus for separating gaseous phosphorus trichloride from a gas stream. The process and process apparatus are particularly useful in preventing or reducing the discharge of gaseous phosphorus trichloride to the atmosphere during the accidental discharge of high volumes of gas from a phosphorus trichloride reaction vessel.
2. Prior Art
One process for making phosphorus trichloride, for example, is to introduce chlorine gas into a layer of liquid phosphorus trichloride containing dissolved phosphorus; the layer resting upon a charge of phosphorus. Chlorination of the phosphorus dissolved in the phosphorus trichloride takes place. Concurrently, with its formation the liquid phosphorus trichloride is distilled off by means of the heat of reaction.
Another process for making phosphorus trichloride is described in British Pat. No. 688,525. This reference describes the introduction of chlorine gas directly into the liquid phosphorus. Phosphorus trichloride vapor is formed by the reaction of chlorine with phosphorus and is led away from the liquid phosphorus and then liquefied by cooling.
Both of these methods of producing phosphorus trichloride result in a gaseous phosphorus trichloride stream. The main product stream is then condensed to liquid phosphorus trichloride by conventional condensing apparatus, for example surface condensers, e.g. air or liquid cooled tubular condensers.
However, during the accidental discharge of a gas stream from a phosphorus trichloride reaction vessel (usually from an emergency pressure relief means), the conventional condensing apparatus utilized to condense the main product stream of gaseous phosphorus trichloride is by-passed thus causing a discharge to the atmosphere of high volumes of gas containing phosphorus trichloride, hydrogen chloride, chlorine and small quantities of phosphorus and inerts.
In order to avoid air pollution problems from such accidental discharges it has been the practice to install, for example, water and/or caustic absorption process equipment to remove the phosphorus trichloride from the gas stream. Such processes are replete with problems. A major disadvantage of such processes is that the phosphorus trichloride is hydrolyzed forming excessive amounts of heat which are difficult to dissipate. Additionally, there are formed products which are difficult to recover and which create secondary waste disposal problems such as water pollution and air pollution problems. For example, the use of the caustic absorption process can result in the formation of phosphine, an explosive gas.
British Pat. No. 688,525 describes a prior art process for removing impurities, such as phosphorus, tarry ingredients and the like, by washing phosphorus trichloride vapor with a liquid phosphorus trichloride. This prior art process, however, does not result in the condensation of any of the phosphorus trichloride; condensation being performed by other methods. Additionally, this prior art process is not utilized to prevent or reduce the discharge of gaseous phosphorus trichloride to the atmosphere during the accidental discharge of high volumes of gas from a phosphorus trichloride reaction vessel.